The above-identified copending patent applications describe the challenges of developing efficient electric motor drives. Electronically controlled pulsed energization of motor windings offers the prospect of more flexible management of motor characteristics. By control of pulse width, duty cycle, and switched application of an energy source to appropriate stator windings, greater functional versatility can be achieved. The use of permanent magnets in conjunction with such windings is advantageous in limiting current consumption.
In a vehicle drive environment, wherein power availability for a traction motor is limited to an on-board supply, it is highly desirable to attain a high torque output capability at minimum power consumption while maintaining high efficiency in all conditions of traction motor operation. Motor structural arrangements described in the copending applications contribute to these objectives. As described in those applications, electromagnet core segments may be configured as isolated magnetically permeable structures in an annular ring to provide increased flux concentration. Isolation of the electromagnet core segments permits individual concentration of flux in the magnetic cores, with a minimum of flux loss or deleterious transformer interference effects occurring from interaction other electromagnet members.
The above-identified copending U.S. application Ser. No. 10/173,610 describes a control system for a multiphase motor that compensates for variations in individual phase circuit elements. A high degree of precision controllability is obtained with each phase control loop closely matched with its corresponding winding and structure. Successive switched energization of each phase winding is governed by a controller that generates signals in accordance with parameters associated with the respective stator phase components and selected driving algorithms. The phase windings are energized with current of sinusoidal waveform for high efficiency operation. The control system varies the output current to respond to, and accurately track, the user's torque command input.
The sinusoidal current waveform profile obtained with this commutation strategy can extend battery life through efficient operation. However, in vehicle driving operation there may be a need for torque capability in excess of that available from the most efficient control scheme. Typically, the power supply is rated for a maximum current discharge rate, for example, 10.0 amps. If the user of the system requests a torque command that correlates to this maximum current draw, then the motor torque output for a sinusoidal current waveform profile is limited, for example, to approximately 54.0 Nm in a motor with a configuration such as described above. In vehicle drive applications, torque input commands are associated by users with commands for change of speed. In typical driving operation, user torque requests are subject to wide variability with little, if any, long term predictability. A driver may demand higher acceleration or greater speed than the system can accommodate at maximum torque with a sinusoidal current waveform. Driving conditions, such as steep uphill grade or heavy vehicle load or the like, may impose other limitations on available speed and acceleration. Other non-vehicular applications may have similar high torque requirements.
The need thus exists for a vehicle motor control system that is capable of performing with high efficiency yet can deliver increased torque output when required by the user. The above-identified Ser. No. 10/290,537 application addresses this need by making available a plurality of motor control schemes for a motor drive, each of which can provide a unique current waveform profile. One of the motor control schemes may be selected by the user to obtain a current waveform profile that has the greatest capability to meet operating objectives. For example, a control scheme may be selected that yields high efficiency operation, such as a sinusoidal waveform, while another control scheme may be selected that provides higher torque, albeit with less operating efficiency. Selection among motor control schemes may be made in accordance with the user's needs or objectives with respect to torque and efficiency, or other factors, e.g., low torque ripple and noise, etc., at any particular time. A selected motor control scheme will be implemented to generate control signals to produce motor energization current having the associated waveform profile.
In a vehicle traction application, for example, user profile selection provides a vehicle driver flexibility to adjust operation to meet objectives. For example, if the user seeks to reach the destination in minimum time, a high torque profile can be selected and maintained throughout a trip to provide maximum speed and acceleration capability. If, however, a greater concern is to conserve an on-board energy source for a relatively long trip, the high efficiency profile can be selected throughout, possibly with the user's selection of the high torque profile at various points on a limited basis. Reference is made to application Ser. No. 10/290,537 for a more detailed description of exemplified waveforms, particularly high efficiency, sinusoidal waveforms, and high torque, square wave shaped waveforms.
The variable conditions and changing requirements of vehicle operation, however, may call for a change in profile more frequently or rapidly than the driver can, or would desire to, keep pace with. A driver's torque requests may be adequately met with selection of the high efficiency profile mode except for relatively transient instances such as passing situations, uphill grades, etc. In those instances, the driver may not be sufficiently responsive to the changing conditions to obtain optimum advantage of a change in selection from a high efficiency profile to a high torque profile. When the high torque requirement conditions diminish, return to the high efficiency profile may be delayed until the user realizes that the high torque profile is no longer necessary, thus drawing unnecessary current from the battery. Thus, it would be desirable to use the high torque mode only when torque greater than that available from the high efficiency mode is required.
The above identified pending Maslov et al. Ser. No. 10/353,075 application describes a system in which a motor control scheme is automatically selected on a dynamic basis to provide an appropriate energization current waveform profile. The system is responsive to a user input signal that represents a torque request. The user input signal is continually sensed and the ability of the system to meet the torque request is monitored to select the appropriate motor control scheme accordingly. The ability of the system to meet torque request is a function of motor speed, which is also continuously sensed to facilitate the torque demand monitoring function. The high efficiency motor control scheme is implemented unless the corresponding current waveform profile is unable to meet the torque demand. The high torque motor control scheme is implemented only when increased torque demand is needed.
While such a system is adaptive to provide appropriate stator current waveforms to meet system torque demands, other conditions, of which the driver may not be aware, may mitigate against implementation of the high torque motor control scheme. Events not directly related to torque production capability may occur that would indicate that the high efficiency mode is appropriate even during periods of high torque demand. For example, the battery supply voltage may reach a low threshold level that represents an impending need for battery recharge or a replacement. The high efficiency profile mode would delay these requirements. As another example, the motor may be operating at severe load conditions, thereby developing increased heating. Motor temperature may reach a level indicative of the need for low current draw and thus an overriding preference for implementation of the high efficiency profile mode of operation. Other external conditions may also factor into a preference for one or more of a plurality of different stator current waveform profiles.
The need thus remains for a more adaptive system in which a motor control scheme is automatically selected based on a plurality of conditions to provide an appropriate energization current waveform profile.